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RVCC Fall 2008 CHEM 103 – General Chemistry I. Chapter 9: Molecular Structures. Chemistry: The Molecular Science, 3 rd Ed. by Moore, Stanitski, and Jurs. Molecular Structure. Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space.

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Chapter 9: Molecular Structures


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    1. RVCC Fall 2008 CHEM 103 – General Chemistry I Chapter 9:Molecular Structures Chemistry: The Molecular Science, 3rd Ed. by Moore, Stanitski, and Jurs

    2. Molecular Structure Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space. Physical and chemical properties depend on the geometry of a molecule.

    3. Molecular Structures 3-D Model 3-D Drawing

    4. Does it matter?The Thalidomide Story  The chemical structure of thalidomide. models – enantiomers (mirror image) “The Same and Not the Same”, by Roald Hoffmann 1995, Columbia University Press

    5. Does it matter?Fatty Acids ‘trans’ fatty acid ‘cis’ fatty acid

    6. VSEPR Model The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible to minimize the repulsion between them.

    7. VSEPR Model Electron Pair Geometry – is determined by the number and arrangement of all electron pairs (bonding and lone) around the central atom. Molecular geometry– is determined by the arrangement of atoms (or bonding electron pairs only) around the central atom. H H H N : In molecules with no lone pairs, Electron Pair Geometry= Molecular Geometry

    8. Fig. 9-4, p.383 • AXE shorthand notation: • A - central atom • X - terminal atoms • E - lone pair electrons AX3E0

    9. Predicting Molecular Geometry: VSEPR • Only five basic shapes. • When a lone pair replaces an atom, the molecular geometry changes as well as the angles. # e- pairs 2 3 4 5 6 Fig. 9-4, p.383

    10. Predicting Molecular Geometry: VSEPR • Draw the Lewis structure. • Determine how many electron pairs (bonded and non-bonded) are around the central atom. **Treat a multiple bond like a single bond when determining a shape. • Write the AXE shorthand notation. • Determine the electron pair geometry (**one of the five basic shapes). • If the molecule has lone pairs around the central atom, then determine the molecular geometry. (This is a subset of the electron geometry.)

    11. .. .. Linear (Electron Geometry)Two e- pairs about central atom bond lone Molecular pairs pairsGeometry 2 0 linear 1 1-3 linear The molecular geometry here is the same as the electronic geometry even though there is a lone pair. ‘Two points make a line.’

    12. Cl Be Cl Predicting Molecular Geometry Example 1:BeCl2 AX2E0 1. Draw the Lewis structure 2. Two electron pairs around the central atom. Two bonded and Zero lone pairs. • electron pair geometry = molecular geometry • Geometry is Linear. Bond angle is 180o.

    13. .. .. .. .. .. .. Trigonal Planar (Electron Geometry)Three e- pairs about central atom bond lone Molecular pairs pairs Geometry Model 3 0 triangular planar 2 1 angular (bent) 1 2 linear

    14. :F: B .. .. F: :F .. .. Predicting Molecular Geometry Example 2: BF3 .. AX3E0 Three electron pairs around the central atom. Three bonded and Zero lone pairs. triangular planar (or trigonal planar)

    15. O S O S O O Predicting Molecular Geometry Example 3: SO2 AX3E0AX2E1 Three electron pairs around the central atom. Two bonded and One lone pairs. electron geometry = triangular planar. molecular geometry = bent or angular.

    16. .. .. .. .. .. .. Tetrahedral (Electron Geometry)Four e- pairs about central atom bond lone pairs pairs 4 0 tetrahedral Model 3 1 triangular pyramidal 2 2 angular (bent)

    17. H C H H H Predicting Molecular Geometry Example 4: AX4E0 CH4 Four electron pairs around the central atom. Zero lone pairs. tetrahedral electron pair geometry = molecular geometry

    18. H N H H N H H H Predicting Molecular Geometry Example 5: AX4E0 AX3E1 NH3 Four electron pairs around the central atom. Three bonded and One lone pair. electron geometry = tetrahedral. molecular geometry = triangular pyramidal

    19. H O H Predicting Molecular Geometry Example 6: AX4E0 AX2E2 H2O Four electron pairs around the central atom. Two bonded and Two lone pairs. O H H electron geometry = tetrahedral molecular geometry = angular or bent

    20. Predicting Molecular Geometry Tetrahedral - bond angles Order of increasing repulsion: bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair

    21. 90° .. .. .. .. .. .. .. 120° Seesaw T-shaped Linear Triangular bipyramidal Trigonal Bipyramidal (Electron Geometry)Five e- pairs about central atom The atoms are non-equivalent. Green atoms are axial; blue atoms are equatorial. **Put lone pairs in the equatorial positions.

    22. : : : F : F : : F : : : : : F : P : F : : Predicting Molecular Geometry Example 7: PF5 AX5E0 Five electron pairs around the central atom. Zero lone pairs. electron and molecular geometry= trigonal bipyramidal

    23. : : F : : F : : S : : F : : : F : Predicting Molecular Geometry Example 8: SF4 AX5E0 AX4E1 Five electron pairs around the central atom. Four bonded and One lone pair. electron geometry = trigonal bipyramidal molecular geometry = seesaw

    24. : : : F : : : F Br : : : : F : Predicting Molecular Geometry Example 9: BrF3 AX5E0 AX3E2 Five electron pairs around the central atom. Three bonded and Two lone pairs. electron geometry = trigonal bipyramidal molecular geometry = T-shaped

    25. : : F : : Xe : : : F : : Predicting Molecular Geometry Example 10: XeF2 AX5E0 AX2E3 Five electron pairs around the central atom. Two bonded and Three lone pairs. electron geometry = trigonal bipyramidal molecular geometry = linear

    26. .. .. .. 90° Square planar Square pyramid Octahedral Octahedral (Electron Geometry)Six e- pairs about central atom Equivalent atoms

    27. S :F : : : : : :F: F: F: : :F :F: : : : : Predicting Molecular Geometry Example 11: SF6 AX6E0 Six electron pairs around the central atom. Six bonded and Zero lone pairs. electron geometry = octahedral molecular geometry = octahedral

    28. : : : F : : : : F F I : : : : : : : F F : : Predicting Molecular Geometry Example 12: IF5 AX6E0 AX5E1 Six electron pairs around the central atom. Five bonded and Two lone pairs. electron geometry = octahedral molecular geometry = square pyramidal

    29. : : : F : F : : : Xe : : F F : : : : : Predicting Molecular Geometry Example 13: XeF4 AX6E0 AX5E1 Six electron pairs around the central atom. Four bonded and Two lone pairs. electron geometry = octahedral molecular geometry = square planar

    30. Predicting Molecular Geometry Fig. 9-5, p.391

    31. Predicting Molecular Geometry Fig. 9-6, p.393

    32. Practice • CO2 • SO2 • ClO2- • ICl • ICl3 • ICl5 • GeF4 • SeF4 • XeF4

    33. Bond Angles CHO Give the approximate values for the indicated bond angles. COH OCN HNH

    34. Molecular Geometry Dipole Moment and Polarity Electronegativity (EN) values are used to predict the polarity of covalent bonds. The greater EN, the more polar will be the bond. A polar bond has a dipole or slight separation of charge (from the unequal sharing of bond electrons). [Chapter 8] The polarity of a molecule depends on the sum of all the bond dipoles (vectors). If there is a net dipole for the molecule, than the molecule is polar. A molecule that has polar bonds may or may not be polar. The dipole moment (μ) is a measure of the degree of charge separation or the polarity.

    35. O H H Molecular Geometry Dipole Moment and Polarity d+ d- d- nonpolar, bp=-79C dipole moment, μ = 0 D d- .. .. + Net dipole d+ d+ polar, bp=100C dipole moment, μ = 1.85 D

    36. Molecular Geometry Dipole Moment and Polarity In general, a molecule is polar if: • it isn’t a basic VSEPR shape (symmetrical) Ex:H2O, bent(polar) • or if the terminal atoms/groups in a basic VSEPR shape differ. Ex:CH2Cl2, tetrahedral (polar)

    37. Dipole Moment and Molecular Geometry Molecules that exhibit any asymmetry in the distribution of electrons would have a nonzero net dipole moment. These molecules are considered polar. Non polar VSEPR shape identical atoms Polar VSEPR shape atoms differ

    38. Dipole Moment and Molecular Geometry

    39. PF4Cl PF3Cl2 PF3Cl2 PF5 Molecular Geometry Dipole Moment and Polarity Non polar Atoms differ. BUT can be divided into nonpolar VSEPR shapes: linear + triangular planar + + Non polar VSEPR shape identical atoms Polar Atoms differ. Doesn’t divide into nonpolar VSEPR shapes Polar VSEPR shape atoms differ

    40. F F S : F F .. F F Xe .. F F Dipole Moment and Molecular Geometry SF4 F ClF3 : SeeSaw No symmetry → polar Cl : F T-shaped No symmetry → polar F F XeF2 : XeF4 Xe : : Linear Symmetric → non polar F Square Planar Symmetric → non polar

    41. Molecular Geometry Dipole Moment and Polarity • Which compound is the most polar? • Which compounds on the list are non-polar? CO, PCl3, BCl3, GeH4, CF4

    42. Orbitals Consistent with Molecular Shape Lewis dot + VSEPR gives the correct shape for a molecule. BUT… How do atomic orbitals (s, p, d …) produce these shapes? Valence bond theorydescribes a bond as an overlap of atomic (hybrid) orbitals.

    43. H 1s F 2s 2px 2py 2pz Valence Bond Model H2 HF

    44. Be [He] 2s2 2s C [He]2s22p2 2s 2px 2py 2pz Valence Bond Theory …and, why do we draw the Lewis structures like we do? This works for H2 and HF, but… • Why does Be form compounds? • no unpaired electrons • Why does C form 4 equivalent bonds at tetrahedral angles? • only two unpaired electrons • p orbitals are at 90° to each other (not 109.5°)

    45. Orbitals Consistent with Molecular Shapes • Atomic orbitals (AOs) can be hybridized (mixed). • Sets of identical hybrid orbitals form identical bonds. • # AOs that hybridize = # hybrids orbitals. • s + p sp + sp • s + p + p sp2 + sp2 + sp2 • etc….

    46. 2p 2p 2p Two unhybridized p orbitals 2p 2p 2p Energy, E Promotion Orbital hybridization Two sp hybrid orbitals on Be in BeF2 2s Isolated Be atom 2s sp Hybrid Orbitals AX2E0,Ex:BeCl2, sp hybridization occurs around the central atom whenever there are two regions of high e- density. Two equivalent covalent bonds form (180° apart) LINEAR.

    47. sp Hybrid Orbitals

    48. sp2 Hybrid Orbitals AX3E0,Ex:BF3 The result is THREE equivalent hybrid orbitals, in a VSEPR basic shape of trigonal planar. p. 396

    49. sp3 Hybrid Orbitals AX4E0,Ex:CH4 TETRAHEDRAL

    50. sp3 Hybrid Orbitals AX3E1 (NH3) and AX2E3 (H2O)